If it's Stranger Things, you bet your butt there's gonna be some kind of big ol' monster terrorizing the young citizens of Hawkins, Indiana. In Season 1, it was the aptly-named Demogorgon, who screeched and tore at our heroes with its carnivorous flower face. In Season 2, it was the protean demo-dogs, harbingers of the terrifying Mind Flayer, looming over the town like a giant many-legged spider. The Mind Flayer, of course, never actually went away despite Eleven closing the interdimensional gate, and it's had all this time to come up with a way to break back into the Right Side Up, and its genius solution is… goo.
Yes, Stranger Things nerds, your big theory about the rats this season turns out to be right on the money. The "mall rats" of Season 3 turn out to be actual furry rats that are lured into an old broken-down factory and transformed into carriers of some weird sickness. Unfortunately for Billy, the bad boy big brother from Season 2, he's the first one to get the bite, and the first to get sent to the Upside Down and become one of the Mind Flayer's minions.
But the Mind Flayer has a Plan B involving the bodies of those rats -- at least, what's inside them. Because the rats, you see, they pop! For whatever reason (and this is a problem Stranger Things has had forever, this weird unwillingness to explain why anything on the show works the way it does), the "disease" takes hold of the rats so violently that their bodies turn themselves inside out and dissolve into fleshy goo that squirts itself across the floor and down the drain.